The present invention is concerned with a process for the production of a suspension of cellulose in a tertiary amine-oxide which is present in liquid state, as well as a process for the production of cellulose moulded bodies such as films, fibres and membranes.
For some decades there has been searched for processes for the production of cellulose moulded bodies able to substitute the viscose process, today widely employed. As an alternative which is interesting among other reasons for its reduced environmental impact, it has been found to dissolve cellulose without derivatisation in an organic solvent and extrude from this solution moulded bodies, e.g. fibres, films and membranes. Fibres thus extruded have received by BISFA (The International Bureau for the Standardization of man made fibers) the generic name Lyocell. By an organic solvent, BISFA understands a mixture of an organic chemical and water.
It has turned out that as an organic solvent, a mixture of a tertiary amine-oxide and water is particularly appropiate for the production of cellulose moulded bodies. As the amine-oxide, primarily N-methylmorpholine-N-oxide (NMMO) is used. Other amine-oxides are described e.g. in EP-A - 0 553 070. A process for the production of mouldable cellulose solutions is known e.g. from EP-A - 0 356 419.
In U.S. Pat. No. 4,246,221, an amine-oxide process for the production of spinnable cellulose solutions using as starting material, among other substances, a mixture of cellulose in liquid, aqueous N-methylmorpholine-N-oxide (NMMO) is described. According to this process, a suspension of cellulose in the aqueous amine-oxide solution is prepared in a discontinuously operating mixing device, the mixture being simultaneously heated at reduced pressure, while water is withdrawn and a first solution prepared, which after filtration and post-processing in an extruder is transformed into a mouldable solution. A disadvantage of this known process is the discontinuous preparation of the cellulose suspension in the mixer. This discontinuous initial step makes the amine-oxide process as a whole discontinuous, which is disadvantageous, since any moulding process such as spinning to fibres is best carried out continuously.
From WO 94/28217, a process for the production of a premixture based on cellulose, wherefrom a mouldable cellulose solution may be prepared, is known. According to this process, shredded cellulose and an amine-oxide solution are introduced into a horizontal cylindrical mixing chamber with a rotor having axially spaced stirring elements. The mixture is stirred in the mixing chamber by rotating the rotor at a rate of from 40 to 80 revolutions per minute. Preferably, the mixture in the mixing chamber is kept at a temperature of above 65.degree. C. This process however has the following drawbacks:
1. The process is very time-consuming, since among other things the amine-oxide solution and the cellulose cannot be introduced into the mixing chamber at once, but must be introduced one after the other. PA1 2. Another drawback of the process consists in that it may be carried out only discontinuously, each batch requiring approximately 21 minutes. PA1 3. Due to the discontinuous operation, two such mixers must be operated in order to deliver the cellulose/amine-oxide mixture continuously to a device wherein the cellulose solution is produced and continuously delivered to e.g. a spinning machine. PA1 4. The use of two mixers implies that every device such as storing tanks, conducts, pumps, filters etc. provided between the mixers and the spinning machine must also be present twice, evidently involving a considerable additional technical and financial effort. PA1 5. Another drawback lies in the fact that relatively big amounts of amine-oxide are processed per batch. These big amounts imply a safety risk, since tertiary amine-oxides are unstable and tend to decompose at elevated temperature. PA1 6. Moreover, discountinuously operated mixers are difficult to empty. PA1 (A) said shredded cellulose is contacted with the liquid, aqueous tertiary amine-oxide, a first mixture being prepared, and PA1 (B) this first mixture is spread mechanically as a layer on a surface and transported along this surface while intensively being mixed, until a homogeneous suspension is produced which is removed from the surface, PA1 (C) measures (A) and (B) being carried out continuously.